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The Russian parliament also recommended that Putin recall the country's ambassador to the United States. Russia has objected to what it sees as U.S. support of the pro-European protests and recent U.S. statements cautioning Moscow against intervention in Ukraine.

President Barack Obama warned Russia Friday against military action in Ukraine. There was no immediate reaction from the White House on the possible recall of the Russian ambassador.

Russia’s parliament speaker told Ukraine’s leader that Moscow could respond militarily if Ukraine uses force against citizens in eastern Ukraine and Crimea. Many people there hold Russian passports, and identify ethnically or linguistically as Russian.

Meanwhile, Ukrainian Prime Minister Arseny Yatsenyuk said that while they hoped for a political solution to the crisis, an invasion by Russia would lead to war.

Vitali Klitschko, a senior member of parliament and one of the main figures in the opposition demonstrations that lead to Yanukovich’s ouster, called for the mobilization of Ukrainian forces.

The Ukrainian Parliament will hold an emergency session Sunday morning.

Ukrainian military sources told Russia’s Interfax news agency that two Russian anti-submarine warships had appeared off Ukraine’s coast near the Crimean port of Sevastopol, reportedly violating an agreement on the naval base.

Need for 'de-escalation'

Putin's open assertion of the right to deploy troops in a country of 46 million people on the ramparts of central Europe has created one of the biggest confrontation between Russia and the West since the end of the Cold War. On Saturday, and world powers scrambled to respond.

Obama held a 90-minutes phone call with the Russian president on Saturday.

Obama told Putin of his “deep concern” over Russia’s military action, according to a White House statement. Obama said that if Russia was concerned about the fate of ethnic Russians in Ukraine, he should directly engage with Ukraine’s government instead of using force. The president said the U.S., as well as European countries, could help facilitate talks between Ukraine and Russia.

Obama’s push for international cooperation was backed by stern warnings. The administration said that the U.S. had suspended its preparatory meetings for the G-8, which is scheduled to take place in Russia, over Russia’s “violation of international law.” The White House said it would cut off other diplomatic ties to Russia if military intervention continued.

Secretary of State John Kerry reiterated U.S. support for Ukraine's interim government and called for Russia to honor its treaty-bound obligations to respect Ukraine's sovereignty. Kerry also urged the Government of Ukraine to continue to "make clear ... its commitment to protect the rights of all Ukrainians."

EU foreign affairs chief Catherine Ashton said Russia's authorization of force was an unwarranted escalation and called on Moscow not to send troops.

Czech President Milos Zeman recalled the Soviet invasion of Czechoslovakia in 1968.

Canada said it was recalling its ambassador from Moscow for consultations in response to Russia's military moves.

The United Nations Security Council held an emergency meeting in New York on Saturday afternoon to review the crisis, and European foreign ministers said they will meet in Brussels on Monday.

British Foreign Secretary William Hague will visit Ukraine’s capital city of Kiev on Sunday for talks with the country’s interim government.

Hague called Russia’s move a "potentially grave threat" to Ukraine's sovereignty, independence and territorial integrity, and said the British Foreign Office had summoned Russia's ambassador to discuss the deepening crisis.

French Prime Minister Jean-Marc Ayrault, speaking to reporters at an event in Rome, said: "There is a requirement that must absolutely be respected – the territorial integrity of the country."

Sweden's Foreign Minister Carl Bildt tweeted that Russia's "likely immediate aim is to set up puppet pro-Russian semi-state" in Ukraine’s partially autonomous Crimea region, a strategically located Black Sea peninsula where Russia has heavy popular support and maintains a crucial naval base under an agreement with Kiev.

Bildt also said Russia's move was "clearly against international law.”

President Barack Obama had warned on Friday that "there will be costs" if Russia intervenes with military measures, sharply raising the stakes in the conflict over Ukraine's future and evoking memories of Cold War brinkmanship.

Putin has said military force may be necessary to protect ethnic Russians in Crimea and personnel at the Russian naval base there. But his parliamentary motion on Saturday referred generally to the "territory of Ukraine" rather than specifically to Crimea – raising the possibility that Moscow could use military force in other Russian-speaking areas in eastern and southern Ukraine, where many oppose the new authorities in Kiev.

Russia's Interfax news agency said Saturday that Putin has not decided whether or not to send troops into the country, and a Kremlin spokesman said Moscow hopes there will be no further escalation in the crisis.

Earlier in the day news reports cited Ukrainian Defense Minister Igor Tenyukh as saying Russia had already sent some 6,000 troops into Crimea, but their presence could not immediately be confirmed.

Russian supporters

Dozens of pro-Russian gunmen in full combat gear patrolled outside the seat of power in Crimea's administrative center Simferopol, a day after similar gunmen seized control over airports and government buildings in the territory.

Many in Crimea say they would support the potential presence of Russian forces there, with demonstrators chanting "Russia! Russia!" in the streets of Simferopol.

In Crimea, pro-Russian regional Prime Minister Sergei Aksyonov had earlier claimed control of the military and police there and asked Putin for help in keeping peace. A planned referendum for greater autonomy in Crimea has been moved up to March 30.

Russia also has many supporters throughout southern and eastern Ukraine, where most people speak Russian and have close cultural and economic ties with their giant neighbor. Violent clashes broke out on Saturday between backers of Moscow and those of the new government in Kiev, who fought each other in the streets of the eastern city of Kharkiv, injuring 97 people.

In Donetsk, the main industrial city in the east, pro-Russian activists seized the building of the regional administration and lifted the Russian flag, according to local Ukrainian media.

Most Ukrainians in the country’s north and west are wary of Russian influence, and culturally and ideologically lean toward the West and the European Union. The country’s crisis spiraled in December after President Yanukovich turned down a trade deal with the EU in favor of billions of dollars in aid from Russia. His decision triggered massive and ferocious protests in Kiev that lasted through the winter, ending last week in Yanukovich being forced from power and fleeing to Moscow.

Russia has said any movements by its military in Crimea are in line with Moscow’s agreements with Ukraine over the lease of the naval base in the port city of Sevastopol, and Moscow has accused Kiev of trying to destabilize the Black Sea peninsula.

Acting President Turchinov of Ukraine said Friday that Russia was following a scenario like the one before it went to war in 2008 with Ukraine’s fellow former Soviet republic, Georgia, over two breakaway regions. The regions are now fully beyond the control of Tbilisi, Georgia’s capital.